1. Field of the Invention
The present invention relates to a switched ring network system having a path protection, with a self-adjusting facility for recovering from failures in the network, and more specifically, to a path protection switched ring network system and a method of recovery from a failure in the network that provides effective recovery from a failure in a currently used path, i.e., a working path, even when a protection path is being used as an additional working path when the failure occurs.
2. Description of the Related Art
In recent years, standardized multiplex transmission-based digital synchronous networks have been proposed such as SONET (Synchronous Optical Network), which is the standard system of North America, and SDH (Synchronous Digital Hierarchy), which is the international standard system of ITU-T (International Telecommunication Union--Telecommunication Standardization Sector). One feature of such systems is that data is always arranged in a predetermined manner. For example, allocations are made for the transmission of "overhead" information used to operate the network.
In forming a ring, a synchronous network can include a path protection switched ring (hereinafter referred to as a PPS ring). In the PPS ring, a primary working path, which is a path currently being used, and a protection path are used as counter-rotating transmission paths having identical assigned time slots. In the event of a failure in the primary working path, the PPS ring switches from the primary working path to the protection path to thereby ensure the protection of the signals on the line.
If there is no failure in the working path, then both the working path and the protection path will transmit identical signals. Practically, this results in a waste of the protection path. In order to utilize the protection path effectively, therefore, a re-use system is often provided in which the protection path is used as an additional working path, to transmit a sub-signal different from the main signal being transmitted over the primary working path. During re-use, the signal on the protection path is not identical to that on the primary working path, which differs from the typical PPS ring arrangement. That is, there is no protection in the working path. This is a problem because the main signal on the primary working path needs to be saved in the event of a network failure.
In the PPS ring, a plurality of network elements are interconnected in a ring configuration by a primary working path and a protection path which are set up to transmit signals, for example, in a clockwise direction and in a counterclockwise direction, respectively, to thereby form a dual-ring configuration. Identical time slots are assigned to both the primary working path and the protection path for signal transmission. Suppose that, in transmitting data from one network element to any other network element, for example, a clockwise working path and a counterclockwise protection path are formed between the transmitting network element and the receiving network element. In this case, the receiving network element normally receives data via the clockwise primary working path. When degradation due to a failure or the like occurs in the primary working path, on the other hand, switching from the primary working path to the protection path is made so that the receiving element can receive data being sent via the counterclockwise protection path. In this manner, signal transmission is always made via a path which is better in quality. In this case, however, the transmitting network element must transmit the same data on both the working path and the protection path. Thus, the number of actually available paths will be reduced to half of the number of actually existing paths on the ring.
It is the re-use system for such a ring configuration that takes a protection path that is not in actual use and communicates some other signal (subsignal) between network elements over the protection path, using the protection path as a working path for the other signal. Under these conditions, self-adjusting of the main signal on the primary working path to the protection path is impossible because the protection path is unavailable.
However, even when the protection path is used as a second working path, it is desirable to be able to protect the signal on the main line.